Electronic devices, to be serviceable under a wide variety of environmental conditions, must be able to withstand moisture, heat and abrasion, among other stresses. A significant amount of work has been reported directed toward various protective measures to minimize the exposure of these devices to the above conditions and thereby increase their reliability and life. Most of these measures, however, suffer from various drawbacks.
For instance, early measures involved potting electronics within a polymeric resin and, thereby, reducing environmental exposure. These methods proved to be of limited value, however, since most resins are not impermeable to environmental moisture and they generally add extra size and weight.
A second method of protection involves sealing the device within a ceramic package. This process has proven to be relatively effective in increasing device reliability and is currently used in select applications. The added size, weight and cost involved in this method, however, inhibits widespread application in the electronic industry.
Recently, the use of lightweight ceramic coatings has been suggested. For instance, Haluska et al. in U.S. Pat. Nos. 4,749,631 and 4,753,856, which are incorporated herein in their entirety by reference, disclose silicon containing coatings produced by applying solutions of silicon alkoxides or silicon alkoxides and metal oxide precursors, respectively, to an electronic device and then ceramifying in air by heating to temperatures of 200-1000.degree. C. These references also describe the application of other coatings containing silicon carbon, silicon nitrogen or silicon carbon nitrogen onto the initial silica layer for added protection. The ceramic coatings produced thereby have many desirable characteristics such as microhardness, moisture resistance. ion barrier, adhesion, ductility, tensile strength and thermal expansion coefficient matching which provide excellent protection to the underlying substrate.
Haluska et al. in U.S. Pat. Nos. 4,756,977, 4,753,855 and 4,822,697 also disclose light-weight ceramic coatings for electronics obtained from hydrogen silsesquioxane resin (H-resin), H-resin with a metal oxide precursor and H-resin with a platinum catalyst, respectively. The process comprises dissolving the H-resin, H-resin and metal oxide, or H-resin and the catalyst in a solvent, evaporating the solvent, and ceramifying in air at temperatures in the range of 200-1000.degree. C. The silica coating may be further coated with additional layers of silicon carbon, silicon nitrogen or silicon carbon nitrogen.
As noted, the above ceramic coating processes involve heating the electronic devices to temperatures of 200-1000.degree. C. in an atmosphere containing oxygen. When certain metals within the devices are exposed to these conditions, however, they begin to deteriorate through oxidation. For instance, when copper is heated to temperatures above 200-250.degree. C. in the presence of oxygen, it oxidizes to CuO which destroys the integrity of the metal and, thus, the electronic device. Moreover, if one attempts to avoid the above destructive effects by keeping the temperature below the oxidation threshold, ceramification takes so long that the process is not commercially feasible.
Wagner et al. in Industrial and Engineering Chemistry, 44(2), 321-326 (1952) reported heating H-resin (silicon oxyhydride) at temperatures as low as 350.degree. C. in an inert atmosphere and observing hydrogen evolution. This reference, however, does not disclose the utility of applying a coating to a substrate under said conditions.
Glasser et al. in J. Non-crystalline Solids, 63 (1984), 209-221, reported applying a nitrided silica coating on a silicon wafer by heating a hydrolyzed solution of tetraethoxysilane in a flowing nitrogen or ammonia atmosphere. This reference, however, discloses that a significant amount of nitrogen from the atmosphere was incorporated into said coating.
Haluska et al. in U.S. Pat. No. 4,826,733 teach the formation of silicon nitride ceramic coatings on electronic devices in an oxygen free environment. The process disclosed therein is a low temperature method whereby a planarizing or passivating coating is formed by dissolving a silicon nitride compound in a solvent, coating the substrate with said solution, evaporating the solvent and ceramifying by heating to temperatures of 200-400.degree. C. in the absence of air. This reference, however, does not teach the use of silicon compounds other than those containing nitrogen.
The present inventors have now discovered that they can apply a ceramic or ceramic-like coating from materials selected from the group consisting of hydrogen silsesquioxane resin or hydrolyzed or partially hydrolyzed R.sub.x Si(OR).sub.4-x, wherein R is independently selected from the group consisting of alkyl, aryl and unsaturated hydrocarbons and x is 0-2, in an inert gas atmosphere at temperatures of 500-1000.degree. C.